The subject invention is generally directed to holographic head-up displays for vehicles, and is directed more particularly to a holographic head-up display having superimposed holographic images.
Head-up displays are utilized in vehicles such as automobiles to produce virtual images of vehicle operating parameter indicators that are ahead of the vehicle windshield and are therefore viewable by operator without diversion of his or her eyes to an instrument panel inside the vehicle. Such indicators status indicators sometimes called "tell-tales" that indicate low fuel, high coolant temperature, low oil pressure and so forth.
A known head-up display technique involves utilizing a reflection hologram supported by the vehicle windshield. The reflection hologram contains a recorded image that is reconstructed pursuant to playback illumination that is selectively turned on to display the recorded image. The playback illumination is commonly directed normal to the reflection hologram, so that the portion of the playback illumination that is reflected back by the windshield returns to the playback source opening in the top of the instrument panel instead of illuminating the top of the instrument panel. Illumination of the top of the instrument panel, particularly at night, causes visuible reflections off the windshield that could interfere with viewing the outside scene.
A consideration with the use of playback illumination that is normal to the hologram is the limitation imposed on the number of holograms that can be used, because of the small optimal area for location of the reflection holograms on the windshield.